Pore-Scale Imbibition Studies for Tight Rock Systems

Date
2021-12
Authors
Peng, Xiaolong
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Publisher
Faculty of Graduate Studies and Research, University of Regina
Abstract

Imbibition pervasively exists in many natural and industrial processes, such as oil/gas recovery, hydraulic fracturing, the postinjection stage of CO2 sequestration, printing processes, diagnostic tests in biomedicine. As a dominant mechanism in tight formations, imbibition has been extensively studied across scales to support world energy demands and environmental requirements of carbon neutrality. Many studies indicate that macroscale performances, like the displacement front instability and residual saturation of the non-wetting phase, highly depend on flow behaviors at the pore-scale level. Despite the importance of pore-scale imbibition processes in tight rocks, many previous studies have been conducted lack considering typical pore-throat features in tight rocks or single-phase flow systems. In this thesis, we employ the analytical, experimental, and numerical methods to study pore-scale imbibition behaviors in pore-throat structures with tight rock features for both liquidgas (L-G) and liquid-liquid (L-L) systems. The analytical portion focuses on imbibition behaviors in microcapillaries with varying cross-sections, emphasizing the effects of viscosity ratios on the equivalent capillary radius and different fluid systems. A modified imbibition equation with an equivalent straight capillary is developed for complex capillaries. The results indicate that at the early stage of imbibition (Zx≤0.12Ψ/(Ψ-1)), the modified imbibition equation can be reduced to a power-law correlation, i.e., Zx∝ 1/Ψα t, where α≈1.025. After that stage, the accuracy of using the power-law correlation begins to decrease. In general, it is more suitable to express the imbibition equations in a quadratic equation, especially for liquid-liquid imbibition systems. In the experimental portions, we quantitively investigate imbibition dynamics in single pore geometries that consider 2D pore-scale characteristics of tight rocks with an etching depth higher than the critical geometric criteria for including 3D features. The typical pore/throat features in tight rocks are summarized and guide for designing the 2D micromodel. The combined effects of the capillary number, fluid properties, and pore-throat structures on imbibition performances are comprehensively discussed. The results demonstrate that with the pore-throat aspect ratio increasing, combined effects of the capillary number, pore shape, outlet throat width (i.e., the flow direction in this work) on non-wetting phase entrapments become more significant. After a critical capillary number, the pore-filling fractions are linearly positive related to the capillary number regardless of the pore shape or outlet throat width, especially for tested macropores. Moreover, attempts, challenges, and recommendations for imbibition studies in 2D and 2.5D micromodels are also documented to guide future researchers. For the numerical simulation portions, we investigate the feasibility of the phase-field method (PFM) in reproducing the strong imbibition dynamics in microcapillaries with typical pore-throat features in tight rocks. In general, the results demonstrate that the PFM simulations can successfully match the imbibition behaviors (such as piston-like movement and wetting film flow) in most micro and mesopores. However, they are challenging to simulate pore-throat structures with an aspect ratio higher than 100 because of convergence problems. The simulation results also agree with the critical contact angle for precursor film flow and find a new type of capillary barrier related to pore shapes in strong imbibition processes.

Description
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Petroleum Systems Engineering, University of Regina. XXIX*, 240 p.
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